This invention relates to an improved pyrohydrolysis process for spent and waste materials generated in electrolytic aluminim reduction sytems. More particularly, it concerns a pyrohydrolysis process wherein fluidization of the fluoridic waste material charge is accomplished by use of an O.sub.2 -containing gas having an O.sub.2 content of at least about 90% by volume.
Pyrohydrolysis of spent and waste materials generated in electrolytic aluminum reduction systems has been described in detail in copending application Ser. No. 855,506 now U.S. Pat. No. 4,113,832 to Bell et al. The process described involves the pyrohydrolysis of spent aluminum reduction cell linings and other fluoridic waste materials, such as floor sweeping, channel cleanings and spent alumina from reduction cell offgas scrubbing systems, in a fluidized bed reactor. Fluidization and pyrohydrolysis generates an NaF and HF-containing offgas from which NaF is recovered and the NaF-free gas, containing the Hf constituent, is used for AlF.sub.3 manufacture and/or production of anhydrous HF. The clinker product of the pyrohydrolysis is essentially Na.sub.2 O.xAl.sub.2 O.sub.3, which is utilized as a source of Al.sub.2 O.sub.3 and Na.sub.2 O, for example by the well-known Bayer process. In this process both NaF and HF are produced, however, the HF content of the offgas is relatively low due to the simultaneous generation of NaF and also since air is used as a fluidizing medium for the spent material charge. Due to the diluteness of the HF in the gas, the AlF.sub.3 product, which is made by contact of HF with Al.sub.2 O.sub.3 in a separate fluidized bed reactor, has an AlF.sub.3 content in the neighborhood of about 13-20% by weight.
In copending application Ser. No. 910,416 to Andersen et al, filed May 30, 1978, an improvement on the above discussed process is described. In the Andersen et al application, the pyrohydrolysis reactor, where the fluidization and pyrohydrolysis take place, is provided with an "extended" reaction zone in the freeboard area of the reactor. The extended reaction zone, in which vapor-phase Na-containing compounds, such as NaF and Na.sub.2 O, are contacted with a relatively finely divided source of Al.sub.2 O.sub.3 in the presence of steam, allows essentially complete conversion of the vaporized NaF to HF and also the formation of Na.sub.2 O.xAl.sub.2 O.sub.3 by the extended reaction of the vaporized Na.sub.2 O with the Al.sub.2 O.sub.3. This improved process provides an essentially NaF-free offgas in which the HF content is significantly increased due to the conversion of the NaF constituent to HF in the extended reaction zone. This higher HF content in the offgas renders the offgas more suitable for the manufacture of anhydrous HF and/or AlF.sub.3 than previous processes. However, the HF is still considerably diluted due to the CO.sub.2 -content of the offgas which has been generated by the combustion of the carbon content of the waste materials, the excess water vapor required to drive the pyrohydrolysis reaction to completion and the large volume of nitrogen introduced with the air used for combustion.
It has now been found that the HF content of the pyrohydrolysis offgas can be further increased by a considerable degree using as fluidizing medium and as a source for combustion of the carbon content of the waste material charge, a stream which contains at least about 90% by volume O.sub.2. Using the essentially nitrogen-free stream for these purposes, in combination with the expanded reaction zone concept where a relatively finely divided source of Al.sub.2 O.sub.3 is contacted with the vaporized Na-components of the offgas, results in a more than 300% increase in HF content of the Na-free offgas. This offgas, as will be shown hereinafter, can be readily employed for the production of an AlF.sub.3 product containing at least about 85% by weight AlF.sub.3 and/or a highly concentrated HF with a fraction of the energy required in comparison to prior art processes.